In the past, it was customary for clinical chemists to measure biological analytes in serum or plasma by flame photometry, coulometry, or fluorometric titration. Hematocrit, the percentage of blood volume occupied by cells (also known as packed cell volume), is measured in whole blood by micro-centrifugation or cell counting. More recent advances in clinical instrumentation have allowed for simultaneous measurement of biological analytes and hematocrit in a single sample. One type of modern blood analyzer measures biological analytes (such as sodium, for example) by direct potentiometry and hematocrit by conductivity. These instruments vastly improve the speed at which hematocrit levels and the concentrations of biological analytes can be obtained, which can lead to improvements in patient diagnosis and care.
In order to confirm the accuracy of blood analyzer measurements, the instrument must be calibrated before use. Some reference solutions use blood cells or other blood products to approximate physiological hematocrit levels. However, blood products are expensive, must be refrigerated during shipment and storage, and are relatively unstable. Thus, a preferred reference solution would not contain blood products, but would still maintain a conductivity similar to a known hematocrit level.
A reference solution for biological analytes must contain known concentrations of each analyte, while a reference solution for hematocrit must have a conductivity similar to that of blood with a known hematocrit level. However, it is difficult to formulate an aqueous solution with physiological levels of biological analytes (such as sodium, for example) and hematocrit in the same solution, because an aqueous environment that lacks red blood cells is far more conductive than whole blood. Accordingly, an additive, such as inert particles or non-conductive water-soluble chemicals, must be added to achieve the necessary conductivity.
Existing reference solutions include high concentrations of conductivity-reducing additives—often up to 30-40% of the total volume of the solution—in order to achieve a conductivity representative of hematocrit levels in whole blood. However, large amounts of additives can drive up the cost of the reference solution, particularly in the case of relatively expensive inert particle additives. In addition, high concentrations of additives can lead to unwanted side-effects, including interference with other analytes in the reference solution, high viscosity, reduced shelf life, and precipitation during shipment and storage. Furthermore, certain water-soluble chemical additives can permeate some sensors within the blood analyzer (such as an oxygen sensor, for example) and reduce the sensitivity and selectivity of the sensors.
Because of these problems, existing reference solutions cannot effectively calibrate a blood analyzer for both hematocrit and biological analytes simultaneously. As a result, at least two separate reference solutions must be used to calibrate a blood analyzer, which reduces the overall speed and increases the cost of operating such instruments.